1. Field of the Invention
The present invention is directed to a highly active catalyst, which is used for use in a living radical polymerization method, and a polymerization method using the catalyst. More specifically, the present invention uses a catalyst having a typical element (i.e., germanium, tin, or antimony) as a central element in a living radical polymerization method.
2. Description of the Related Art
A radical polymerization method has been a well known method for polymerizing a vinyl monomer to obtain a vinyl polymer. Generally, a radical polymerization method has the disadvantage of the difficulty in controlling the molecular weight of the obtained vinyl polymer. Further, there is the disadvantage that the obtained vinyl polymer is a mixture of compounds having various molecular weights, and thus it is difficult to obtain a vinyl polymer having a narrow molecular weight distribution. Specifically, even if the reaction is controlled, the ratio of weight-average molecular weight (Mw) and number-average molecular weight (Mn), (Mw/Mn), can be only reduced to about 2 to 3.
As a method for eliminating the aforementioned disadvantages, since around 1990, a living radical polymerization method has been developed. Specifically, according to the living radical polymerization method, it is possible to control the molecular weight. It is also possible to obtain a polymer having a narrow molecular weight distribution. Specifically, a polymer having Mw/Mn of 2 or less can easily be obtained. Therefore, this method has come into the limelight as a method for producing a polymer used in a high technology such as nanotechnology.
Catalysts which are currently used in living radical polymerization methods include transition metal complex-type catalysts.
For transition metal complex-type catalysts, complexes in which a ligand is coordinated to a compound having a central metal of Cu, Ni, Re, Rh, Ru, or the like have been used. Such catalysts are described in the following documents for example.    [patent document 1] Japanese Laid-open Patent Publication No. 2002-249505    [patent document 2] Japanese Laid-open Patent Publication No. 11-322822    [non-patent document 1] Journal of the American Chemical Society 119, 674-680 (1997)
Patent document 1 discloses that a complex with a central metal which is Cu, Ru, Fe, Ni or the like, and it is used as a catalyst.
Patent document 2 discloses that hydrido rhenium complex is used as a catalyst.
Non-patent document 1 discloses that a compound in which 4,4′-di-(5-nonyl)-2,2′-bipyridine is coordinated with copper bromide, is used as a catalyst.
However, when such transition metal complex catalysts are used, it is necessary to use a large amount of the catalyst. This is disadvantageous as it is not easy to completely remove the large amount of the catalyst used, from the products after the reaction. Another disadvantage is environmental problems which may occur by the disposal of the catalyst. The transition metal for the living radical polymerization method includes many toxic metals. The disposal of a large amount of such toxic metals causes environmental problems. Furthermore, there are cases where toxicities of catalysts remaining in products cause environmental problems. Due to the toxicity, it is difficult to use the transition metal catalysts for the production of food packages, material for living body, and medical material. Additionally, there is a problem associated with a high conductivity of the transition metal remaining in polymer, rendering the polymer conductive and hence unsuitable for use in electronic material such as resist material. Furthermore, the transition metal-type catalysts do not dissolve in a reaction solution unless they form a complex. Therefore, it is necessary to use a ligand as an additive to form a complex. This causes problems, i.e., an increase of the cost of production and also an increase of the total weight of the catalyst used. Further, a ligand is usually expensive and requires complicated synthesis method.
Living radical polymerization methods, which do not require a catalyst, have also been known. For example, a nitroxyl-type method and dithioester-type method have been known. However, these methods have the following disadvantages. A special protecting group (i.e., a certain nitroxide or dithioester group) must be introduced to the polymer growing chain. The protecting group is very expensive. Further, the polymerization reaction requires a high temperature (for example, 100° C. or higher). Further, the produced polymer is likely to have undesirable properties. For example, the produced polymer is likely to be colored differently from the natural color of the polymer. Further, the produced polymer is likely to have an odor.